1. Field of the Invention
The present invention relates to a solid-state imaging device, such as a CCD image sensor or a CMOS image sensor, including an imaging pixel area defined by two-dimensionally arranged pixels, and color filters.
2. Description of the Related Art
Conventionally, solid-state imaging devices, such as CCD image sensors and CMOS image sensors, are known as miniature cameras to be mounted in digital cameras and portable telephones. Solid-state imaging devices include an imaging pixel area in which multiple pixels, each having a photoelectric conversion region such as a photodiode, are two-dimensionally arranged in rows and columns. Imaging light is converted into signal charges by the pixels, and the signal charges are converted into electrical signals to obtain image signals corresponding to a two-dimensional image.
In CCD image sensors, signal charges obtained by photodiodes in pixels are transferred to an output unit by a CCD transfer register, and are output after being converted into electrical signals in a lump. In CMOS image sensors, each pixel includes a photodiode and a transistor circuit. A signal charge obtained by the photodiode is converted into an electrical signal by the transistor circuit, and is output through a signal line.
In these solid-state imaging devices, in order to capture a color image, for example, RGB primary-color filters are arranged in a mosaic form on an imaging pixel area, a group of adjoining pixels receives light components with wavelengths for R, G, and B, and color image signals are obtained from RGB color-component image signals.
For example, Japanese Unexamined Patent Application Publication No. 2000-201355 discloses a color CCD imaging device using, as a color filter having such a color pattern, a color filter having a so-called primary-color Bayer pattern. In the primary-color Bayer pattern, one red pixel, one blue pixel, and two green pixels are arranged in a matrix of two by two pixels to define one group.
In the following description, four pixels (one red pixel, one blue pixel, and two green pixels) that define one group are referred to as a base unit.
In the primary-color Bayer pattern, two green filters are diagonally disposed in a well-balanced manner in each base unit defined by four pixels. Therefore, this pattern is effective to ensure brightness gradation.
In the above-described solid-state imaging device, when the size of each pixel and the number of transistors in each pixel (in CMOS image sensors) decrease, pixels sometimes have different shapes (structures).
FIG. 6 is a plan view showing the pixel pattern of an imaging pixel area in a known solid-state imaging device having primary-color Bayer color filters.
As shown in FIG. 6, multiple pixels 10A and 10B are basically arranged in a matrix of rows and columns. A primary-color Bayer RGB color filter 20 is disposed correspondingly to each base unit 12 defined by 2 by 2 pixels. In the figure, base units and color filters are enclosed by boxes shown by broken lines.
Pixels 10A marked with A and pixels 10B marked with B are different in structure. Two pixels 10A having an A-structure are arranged on one side (upper side in the figure) of each base unit 12, and two pixels 10B having a B-structure are arranged on the other side (lower side) thereof.
However, when primary-color Bayer color filters are provided in the above-described solid-state imaging device including pixels having different structures, the image quality is sometimes reduced, for example, luminance variations (bands) are found among lines (pixel rows).
It is thought that this is caused by the assignment of pixels having different structures to filters of the same green color in each base unit of the primary-color Bayer pattern.
FIGS. 7 and 8 show examples of pixels having different structures. FIG. 7 shows that the positions of readout transistors in pixels are not the same. FIG. 8 shows a case in which the light-receiving areas of photoelectric conversion regions in pixels are not the same.
In FIG. 7, the position of readout transistors (readout gates) 32 for reading charges from photoelectric conversion regions 31 in pixels 30A is different from that in pixels 30B. For this reason, the pixels 30A and the pixels 30B have different light-receiving characteristics with respect to oblique light.
In FIG. 8, the size of photoelectric conversion regions 41 in pixels 40A is different from that in pixels 40B (the positions of readout transistors (readout gates) 42 are the same). In this case, the pixels 40A and the pixels 40B also have different light-receiving characteristics.